The proposed research will characterize the role of saccade-related cerebellar output from the caudal fastigial nucleus (CFN), in long-term saccade adaptation. Cerebellum is strongly implicated in short-term saccade adaptation, but its role in long-term saccade adaptation is unknown. Long-term adaptation has not been extensively studied, but is essential for maintaining movement accuracy during growth, aging, and for permanent rehabilitation of impaired movements. Long-term adaptation of saccades will be produced by presenting an adapting stimulus to a monkey for many days. In the first experiment, behavioral tests and temporary inactivation of the CFN will be used to test our hypothesis that cerebellar contribution to adapted saccades decreases rapidly after the initial adaptation. In a second experiment, permanent bilateral CFN lesions will be used to test our hypothesis that CFN output drives long-term adaptation. In third experiment, injections of anterograde tracer in the CFN, and retrograde tracer in the frontal eye field (FEF) will test our hypothesis that CFN output reaches the FEF via a relay in the thalamus. This pathway may induce long-term adaptation by modifying saccade commands descending from the FEF. The proposed research has three specific aims: (1) To measure the contribution of CFN to adapted saccades at different times during long-term adaptation. (2) To measure the long-term adaptation after permanently lesioning the CFN bilaterally (3) To determine the anatomical pathway of Iong-term adaptation.